1. Field of the Invention
The present invention relates to a semiconductor device having a pixel portion or having this pixel portion and its driving circuit on the same insulator surface. In particular, the invention can be suitably used in a display unit in which a liquid crystal material is provided between electrodes. In this specification, the semiconductor device generally means a device able to function by utilizing semiconductor characteristics, and the above display device, an integrated circuit and an electronic device mounting the integrated circuit and the display unit thereto belong to a category of the semiconductor device.
2. Description of the Related Art
A matrix driving system (this is called an active matrix driving system) for arranging a thin film transistor (hereinafter abbreviated as TFT) every pixel or dot has become an indispensable technique as high definition of pixel density is advanced. The active matrix driving system using the TFT can prevent crosstalk generated in a simple matrix driving system.
Another advantage of the active matrix type display unit is that an integrated circuit such as a shift register, a latch circuit or a buffer circuit can be formed by the TFT on the same insulator as a driving circuit for transmitting a signal to a pixel portion. Thus, the number of contacts with an external circuit can be very reduced and reliability of the display unit can be raised.
Two kinds constructed by a direct viewing type for directly viewing a picture image displayed in the pixel portion and a projection type for projecting this picture image onto a screen by using an optical system are developed in the display unit using a liquid crystal material. In both these types, segregation is considered on the basis of a screen size such that the direct viewing type is used until about 30 inches in screen size and the projection type is used at a screen size of about 30 inches or more.
A liquid crystal is generally operated by an alternating current, and a frame inversion driving system or a line inversion driving system is adopted. In any case, the TFT is used to control a voltage applied to the liquid crystal. Since resistance of the liquid crystal is high, the TFT requires characteristics such as a sufficiently large ON-state current (showing a drain electric current flowing at a turning-on state time of the TFT) able to charge a pixel capacitor (the liquid crystal itself) during a scanning period, a sufficiently small OFF-state current (showing the drain electric current flowing at a turning-off time of the TFT) able to hold electric charges during a field period, a sufficiently small parasitic capacity between gate and drain electrodes, etc. Since pixel capacity is small and a holding operation is insufficient, an auxiliary capacitor is arranged in a pixel to compensate the pixel capacity and prevent an influence of the parasitic capacity.
In the pixel of the active matrix driving system arranging the TFT therein, a scanning line (gate line) connected to the gate electrode and a data line connected to the source or drain electrode cross each other in addition to a pixel electrode for applying a voltage to the liquid crystal. Two kinds constructed by an additional capacity type for overlapping the pixel electrode and a scanning line (gate line) at the previous stage and an accumulating capacity type for arranging a dedicated capacity line are known in the auxiliary capacitor. In any case, sizes of the TFT and the auxiliary capacitor allowed per one pixel are necessarily reduced as high definition of image quality is advanced. Accordingly, it is indispensable to the obtaining of high numerical aperture of each pixel in the prescribed pixel size that elements required in these pixel constructions are efficiently laid out.
A light interrupting film is an element particularly required in the liquid crystal display unit of a transmission type. A semiconductor film has an optical electroconductive effect in which a resistance value is changed by light irradiation. Accordingly, the semiconductor film has an influence on an OFF-state current by irradiating light from a light source to this semiconductor film. In particular, in the display unit of a projection type, a problem exists in that one portion of light emitted from the liquid crystal display unit is reflected on the interface of a substrate and an air layer and is also reflected in an optical system, and is returned in a reverse direction and is incident to the TFT.
In the case of the projection type with a metal halide lamp, etc. as a light source, light of one million to twenty million lx is irradiated to the liquid crystal display unit so that design of the light interrupting film becomes important. In the display unit of the transmission type, incident light 28 from the light source is incident from the side of an opposite substrate 22 as shown in FIG. 20, and passes through a liquid crystal 27 and is transmitted to the side of an element substrate 21 forming a TFT 23 therein. A light interrupting film 26 is formed on the TFT 23 so that no incident light 28 is directly irradiated to the TFT 23. However, a light component reflected on the interface of the element substrate 21 and the air layer and diffused within the substrate is considered as diffused light 29, and its one portion is incident to a semiconductor film 24. Conductivity of the semiconductor film 24 is raised by the optical electroconductive effect and this raised conductivity has a bad influence on the image display such as an increase in the OFF-state current of the TFT, a reduction in contrast and generation of crosstalk, etc. However, when light interrupting property is preferentially treated and an area of the light interrupting film 26 is increased to interrupt such light, the numerical aperture is naturally reduced.
It is indispensable to the realization of high numeral aperture in the limited pixel size that elements required for the construction of a pixel portion are efficiently arranged. A first object of the present invention is to provide an active matrix type display unit having a pixel structure in which a pixel electrode formed in the pixel portion, a scanning line (gate line) and a data line are suitably arranged, and high numerical aperture is realized without increasing the number of masks and the number of processes.
Further, a problem exists in that manufacturing cost is increased when a manufacturing process of the TFT is complicated in the active matrix type display unit. When the manufacturing process becomes complicated to simultaneously form plural TFTs, yield is reduced. When a fault relative to the manufacturing process is caused in a driving circuit, a linear defect is caused. A second object of the invention is to reduce the manufacturing cost of the active matrix type display unit and provide a cheap display unit. Further, another object of the invention is to provide a cheap electronic device using the display unit of the invention in a display portion.
The construction of the invention is characterized in that a first wiring arranged between a semiconductor film and a substrate through a first insulating layer is overlapped with the semiconductor film and is used as a light interrupting film by this overlapping. Further, a second insulating layer used as a gate insulating film is formed on the semiconductor film, and a gate electrode and a second wiring are formed on this second insulating layer. The first wiring and the second wiring cross each other through the first and second insulating layers. In a pixel portion, the first wiring functions as a scanning line (gate line), and the second wiring functions as a data line. A third insulating layer is formed as an interlayer insulating film on the second wiring. A pixel electrode is formed on this third insulating layer. The pixel electrode can be overlapped with the first wiring and the second wiring so that an area of the pixel electrode can be increased in the display unit of a reflection type.
In another construction of the invention, a first wiring and a third wiring are formed on the substrate, and a first insulating layer, a semiconductor film and a second insulating layer used as a gate insulating film are sequentially laminated on the first wiring and the third wiring. Similar to the first construction, a gate electrode, a second wiring, a third insulating layer and a pixel electrode are formed on the second insulating layer. In the case of the display unit of the reflection type, the pixel electrode can be overlapped with the first wiring, the second wiring and the third wiring so that an area of the pixel electrode can be increased and numerical aperture can be improved.
In the above first and second constructions, the semiconductor film is light-interrupted from its both faces by the first wiring and the pixel electrode in the case of the display unit of the reflection type. Such a construction can be preferably used in a liquid crystal display unit particularly assembled into a projecting apparatus of a projector.
Such a pixel structure of the invention can be preferably used in the liquid crystal display unit. In particular, numerical aperture can be greatly improved in the liquid crystal display unit of the reflection type. Further, this pixel structure can be also applied to an EL display unit (a light emitting device or a light emitting diode or OLED (Organic Light Emission Diode)) of an upper face radiating type. The EL (electroluminescent) devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example.